To the Editor: In their single-blind crossover study, performed in a hypobaric chamber, Dr Toff and colleagues1 did not find relevant prothrombotic alterations in healthy individuals after 8 hours of sitting under simulated hypobaric hypoxic conditions. Data were similar after sitting under normobaric normoxia. The authors rejected the hypothesis that hypobaric hypoxia of the degree that might be encountered during long-haul air travel is associated with prothrombotic changes in the hemostatic system in persons at low risk for venous thromboembolism.